Thermionic cathode structure



March 4, 195% c. c. CUTLER 9 5 THERMIONIC CATHODE STRUCTURE Filed Dec.3,1953

FIG. i

//v VENTOR C. C. CU TL E R A T TOR/VEY THERMIONI CATHODE STRUCTURECassius C. Cufler, Gillette, N. J., assignor to Bell TelephoneLaboratories, Incorporated, N ew York, N. Y., a corporation of New YorkApplication December 3, 1953, Serial No. 396,027

7 Claims. (Cl. 313-38) This invention relates to thermionic cathodes andmore particularly to mounting the cathode structure in a high frequencythermionic electron discharge device.

The operating characteristics of high frequency electron dischargedevices are dependent to a large extent upon the inter-electrodespacings. In order that a particular device have predetermined operatingcharacteristics it is necessary that the inter-electrode spacings beaccurately established during fabrication of the device and bemaintained constant during operation of the device. This is necessarysince the spacings are extremely small, and changes of small absolutemagnitude therein result in relatively large variations in the operatingcharacteristics. All this is particularly true of the cathode and itssupporting structure. In addition the cathode should be thermallyisolated from the other elements of the device so that the cathode maybe heated to the temperature required for thermionic emission withoutundue power expenditure.

These several requirements have been met in prior devices by eithercomplicated cathode supporting structures or by structures made ofmaterials having different temperature coeflicients of expansion orboth. An attempt is made in the latter structures to compensate in oneportion of the supporting structure for the heat pro duced expansion inanother portion. This compensation is complicated by the fact that therange of temperatures over which the device is often called upon tooperate will vary with the supply voltage producing the heating. It isfurther complicated by the fact noted above that the component size andspacing are very small. In addition, the preparation of the supportingstructure involvesfastening the supporting components together and tothe cathode by some means, most often by welding, and often involvespre-forming the cathode to a particular shape in manufacture.

It is therefore an object of the present invention to mount an electronemitting cathode with respect to other elements of an electron dischargedevice so that the spacing between the cathode and the other elements ismaintained substantially constant regardless of the temperature of theemitting cathode.

It is a further object of the invention to dispose and mount an electronemitting cathode more accurately and simply during manufacture than ispossible with prior mountings.

A more specific object is to provide a mounting for a cathode in adischarge device of the type requiring the production of an accuratelycontrolled electron beam which eliminates the necessity of specialpre-forming of the cathode structure or of welding a supportingstructure to the cathode, while at the same time maintaining a highdegree of heat concentration at the cathode.

In accordance with one embodiment of the invention to be described, thecathode takes the form of a flattened disc having a plurality ofapertures spaced near its periph' eral edge. This edge is looselyreceived within a cavity formed by rigid support members, which may, inthe te ates atefii ice case of the cathode ray tube, comprise in partthe focusing electrode itself. Thin, wire-like elements of low heatconductivity material are looped through each of the apertures to bearagainst the inside of the cavity. Thus, as the cathode is heated it isfree to expand in a radial direction but is restrained at asubstantially constant distance equal to the diameter of the wires fromthe supporting structure. Furthermore, since the cathode is touched at aminimum number of points, and then only by the low heat conductivitywires, little heat conduction is experienced away from the cathode. Inanother embodiment of the invention the loops are replaced by helicalmembers of material similar to the wires located on either side of thecathode within the cavity.

These and other objects, the nature of the present invention, itsvarious advantages, and its features will appear more fully uponconsideration of the various specific illustrative embodiments shown inthe accompanying drawings and described in the following detaileddescription.

In the drawings:

Fig. 1 is a cross-sectional view of a portion of an electron dischargedevice, in accordance with the invention, showing the mounting of thecathode and illustrating the relationship of the cathode to otherelements of the device;

Fig. 2 shows in perspective view the cathode portion of the assembly ofFig. 1;

Fig. 3 is a cross-sectional viewof an alternative cathode shape inaccordance with the invention;

Fig. 4 is a cross-sectional view of a second embodiment of the inventionillustrating a modification of the cathode support of Fig. 1; and

Fig. 5 shows in perspective view the cathode portion of the assembly ofFig. 4.

Referring more specifically to Fig. 1, an illustrative embodiment of theinvention is shown in the form of an electron gun of a beam-type tubesuch as a cathode ray tube or a traveling wave tube. Enclosed withinportion 11 of the evacuated outer casing or bulb of the tube, andsupported therein by suitable means is an anode ring 12 of conventionaldesign having an aperture 13 in the center thereof to allow for passageof the produced electron beam to portions of the tube not shown which,however, would be located above the structure shown in Fig. l. Anode 12is biased at a high positive potential relative to focusing electrode 14as indicated by the conductive leads marked B+ and B.

Electrode 14 may be as illustrated, an element of conventional designcomprising a ring of conductive material having a generally conicaltaper extending toward the center opening 15 thereof and in a directionaway from anode 12. Electrode 14 is separated and supported from anode12 by ring 16 of insulating material, such as a suitable ceramic orporcelain material.

Coaxially arranged with aperture 13 of anode 12 and aperture 15 ofelectrode 1 1 is the cathode assembly in accordance with the invention.This assembly comprises an emissive layer 17, such as an oxide of one ofthe rare earths, for example, barium or strontium. Layer 17 is carriedby a flattened disc or button-like plate 18 of a high melting point,high heat conductivity, electrically conductive material such as nickelor iron. integrally connected to plate 18 and coaxially arrangedtherewith is a tubular heat shield 19 of material similar to plate 18.Shield 19 surrounds a conventional tungsten heating element 20.

Located around the periphery of plate 18 and displaced toward the centerfrom the edge thereof by a convenient distance, are a plurality ofapertures 21 extending through the thickness of plate 13. This is most'clearly' seen in the perspective view of Fig. 3. If, for

. t aeeasee e example, the diameterofplate; 18 is in the order of'.150

' shown in Fig.' 5. A plurality of helical members 31 inch a convenientdistance from the edge th ereof. for

Before assembling :the cathode structure.aashortt length'g.

of wire-like element 22 of material having: a low; -co-. eificient of;heateconduetivity and a high melting .point .:is

looped through each;,of ;ap,ertu res .21 Suitabletmatcrials for wires22-may be. tantalum,,stainless stealer-other. lowheat conductivitymaterial; However: nonmetallic materials, may also bQMSEd-ZQSWllltbB.described herein after, Preferably, three such. aperture-wire.combinations I may-be employed, butmoretmay; satisfactorily be.pro

videdifdesired After wires; 22i.are1 inserted as=described, plate 18. islocated elative to aperture of; electrode 14 asdefined above so thatwires-.22.:zbear against the lower. surface:

of -electrode:.14,; as shown. Thus,:wires- 22 provide a separationbetween electrode. 14.and plate 18 substantiallyequal to the dimeterofwires22 and this dimension-- may. be critically selected byproperchoiceot the wirediameter, Upon this, assemblyis placed a clampingringample, smallmachine bolts. extending into tapped holes.

in electrode 14. Thus, recess 24 together with the-lower face. ofelectrode14 forms a cavity =within-whieh wires 22 are pressed againstthe inside surfacethereof; The

slight resilience of wires 22 will hold plate 18 securely at asubstantially constant distance from-focusing electrode 14..As.thecathode is heated, however, plate 18 is free to expand laterallyin a radial direction. In order to accommodate this expansion thediameter of recess 24 should be substantially equal to the heateddiameter of I plate 18 plus twice the diameter of wires 22. Therefore,upon heating the cathode thev emissive layer 17 thereof on plate 18 willbe substantially centeredwith respect to aperture 15. Since the onlyphysical contact between plate 18' and its supporting structurecomprising ring 23 and electrode 14 is through the point-like contact ofthe several low heat conductivity Wires 22, little heat'transfer isexperienced away from the cathode structure 1718--19.

In the embodiment illustrated cathode 17 1819 is operated at thepotentialof electrode 14 as is common in cathode ray-type tubes.However, if electrical'isolatiou is desired, wires 22 may be made ofdrawn quartz strings or other insulating material having a low heatconductivity and a suitable amount of resilientfiexibility,Alternatively, either ring 23- or a portionof electrode 14 or both maybe of insulating material, such as a suitable ceramic material. Also,the present cathode Support is equallysuitable for use in an electronvdischarge device employing a control grid. In this application aconven-- tional support of the control grid replaces electrode 14 in.the structure illustrated. Furthermore, the cathode may be heated byelectron bombardment in which case. the principles described in UnitedStates Patent 2,509,053, granted May 23, 1950, to 0 Caloick may beemployed.

The plane shape of cathode, plate lSismerely shown by way of example.The form thereof is susceptible to variation as shown in Fig. 3, inwhich a plate 26.having apertures 28 isprovidedwtih a. raised portion inthe cen-.. ter thereof which carries the electron emissive coating 27.

In Fig 4 an alternative embodiment of the invention is shown as amodification of the structure of Fig. 1 Withcorresponding referencenumerals. employed to desige nate similar components. Modification will;be seen to reside in the cross-sectional shape. of cathode plate 29which is now provided wtih a tongue-type eutensiongl) from itsperiphery. A perspective detail'of plate-29 is each composed of several.turns. of fine -.-.wire-0f--materialsimilar to the wires 22 of Fig. 1are disposed at spaced locations around the circumference of plate 29.Thus members 31 bear upon the inside surface of cavity 24 formed by therecess between tongue 30, plate 29 and the corner between clamp 2 3-,-and;electrode 14. A similar plurality of helical members 32 areplaced on the other side of tongue..30. withinthewcavity 33- formed bythe cessinla nu i. e-fi lsandrp ta 9- H i al members 31 and L er ea simlae uns anl.w hs mflaradvantages, to wire elements 22 of,Fig..l.'

In all cases, itis understood that the above described arrangementsaresimply illustrative of the small number of the many possible;specifieembodiments which can represent applications of the principles of theinvention. Numerous and varied other arrangements can readily be devisedin accordance with these principles by. those. skilled inlthe artwithoutdeparting from the spirit andscopeof theinvent'ion;

What is claimeddst 1 11A cathodestructure for electron dischargejdeyicescomprisinga flattened cathode member having anelec: tronemiss'i-v'esurface-extending-in a transverse direc' tion at-leasttwo-rigid-supporf members with means}. for-constraining said membersrelative to, one another in} a longitudinal direction, said m'embersfo'rming .acavityj to loosely-receive said cathode around the peripheraledge, of said-cathode, anda plurality of thin wire, element's interposedlongi-mdinallyand transversely betweensaid support members and said'cathode, saidelements' beaningat leas't in; a-longitudinaldirectionagainst the insideof said-cavity.

2; The-combination of claim- 1 wherein said cathode memberhas' aplurality of apertures extending through the thickness thereof, andwherein each ofl sai'd elementlsgl comprises a loop of low heatconductivity material passing through each of said aperturesg 3. Thecombinati'on of clainijl wherein each .of ,said elements comprises ahelix of low he'at co'nd'uctivit'y ma terial a portion of saidfelementsbeing located wi'thin, 7 said cavity on one side of said cathodeand the; remainder of said elements being" located within said} cavityon .the... other side-of said'cath ode'.

4. In an electron discharge device ofthetype' haying; a rigidelectrodewithreg'ard to which .a flattened vtherrnic thvdei to PP td an-d. sp c:1 embe having. an annular recess deeperj than the. thickness. -of aihod andwid'e n. e ne m n ennfisai da d memb r ssu o t o s sa d.electrode; with said cathode disposed .withinsaid, recess; said. atho e;vi .,-a' l l ty f ap r e -e amine, through the thickness thereof, andawire-lilgeloop of low r. heat conductivity. material ,passingthrougheach: of said; apertures to bear against said .electrodepn oner side ofsaid" cathode and against, said; member on 'the othen-side -n f idca hde- 5. In. an. electron discharge .,,device, a fiattened dise;v of highheat conductivity material. havinga-coatingmf-g thermionic material atubular shield of highgheat con-; ductivity. material having oneend,.:coaxially: connected;:.-- to said. disc, a thermionic heating!element located-with. in said' shield to raise the temperature of saidcoatingltoi. the temperature of thermionic emission, rigid support--members. forming auannular cavity to loosely receive said discj around.the peripheral edge of said-disc, said-disc having. a: pluralityof,aperturesgextending through; the. thickness thereof. and spaced;around the-peripheral edge thereof, and athin wire; element of, lowheat. conductivity. materialllooped around ,tl e,of said disczand-through; each ofg'said apertures; disc ying ;a -heated;diameter-.substantiallvequahto sof- SaldfCaV lCY-"ICSS se-fl edie e efi a d a v snaa ness, substantiall egu widthiofgsaidcavity-less ces es iensl a te einherit.

6. In an electron discharge device, a flattened disc of high heatconductivity material having a coating of thermionic material extendingin a transverse direction, a tubular shield of high heat conductivitymaterial having one end coaxially connected to said disc, a thermionicheating element located within said shield to raise the temperature ofsaid coating to the temperature of thermionic emission, said disc havinga tongue-like extension extending out from its periphery, at least tworigid support members with means for constraining said members relativeto one another in a longitudinal direction forming an annular cavity toloosely receive said tongue, and a plurality of helical members of lowheat conductivity material located on both sides of said tongue, saiddisc excluding said tongue having a diameter substantially equal to thediameter of said cavity less twice the diameter of said helix, saidtongue having a thickness substantially equal to the width of saidrecess less twice the diameter of said helix so that said helicalmembers bear at least in a longitudinal direction against the inside ofsaid annular cavity.

7. A cathode structure for electron discharge devices comprising aflattened cathode member having an electron emissive surface extendingin a transverse direction, a plurality of thin wire elements each ofsaid elements having transversely extending portions in contact withsaid cathode, and having one longitudinally extending portion spacedfrom said cathode, at least two rigid support members forming a cavityto loosely receive said cathode around the peripheral edge of saidcathode, means for constraining said members relative to each other in alongitudinal direction with said members hearing upon each of said wireelements in a longitudinal direction and otherwise forcing said onelongitudinally extending portion of each of said elements tosubstantially conform transversely to the inside of said cavity.

References Cited in the file of this patent UNITED STATES PATENTS2,154,275 Linn Apr. 11, 1939 2,310,811 Schantl Feb, 9, 1943 2,413,689Clark Jan. 7, 1947 2,421,767 Varian June 10, 1947

